A nuclear reactor, for example, a pressurized-water reactor, normally comprises a pressure vessel containing the core and having a removable cover for the purpose of permitting refueling of the reactor core within the vessel. Measuring instruments are positioned within the core and have measuring lines conducting electrical or physical or measuring values from the core and through a suitable passage in the pressure vessel, normally in its cover, and leading to instrumentation for determining the conditions existing in the core within the vessel.
These measuring lines extending from a plurality of sensor units within the core, must be bundled and passed in a pressure-tight manner through the pressure vessel and to external instrumentation. The various measuring lines can be bundled so they can be passed through a single opening in the pressure vessel, normally through the vessel's removable cover. When done in this manner, removal of the cover of the pressure vessel, such as is done for refueling of the core in the vessel, results in withdrawal of the measuring lines and their sensor units from the pressure vessel.
Complete pressure sealing between the bundle of measuring wires and the pressure vessel is, of course, essential. Furthermore, in the event one of the sensor units, or probes, must be replaced or serviced when the pressure vessel cover is removed, the individual measuring line for that sensor unit should be removable relative to the cover.
The measuring lines have relatively thin-walled cladding which is mechanically and thermally sensitive, and they are often connected undetachably with their sensing or measuring devices. Sealing of the clad measuring lines relative to the vessel, such as its cover, therefore presents substantial difficulties. If one of the measuring lines and its sensor unit only must be replaced, such individual replacement is desirable, as contrasted to replacing the entire bundle of measuring lines which pass through the reactor pressure vessel to the external instrumentation.